Builders SAVE15 voucher never works

Builders SAVE15 voucher never works on the selected advertised products. I had never understood false advertising by this retailer. They had said its only valid once, What false advertising is this if we had bought from the retailer weeks ago other advertised products and now cannot get a discount on the current advertised product. Why cant they state this on the promo? Why is this still featured on the Website? Really Builders how can you?